This invention is concerned with plating highly reactive metals, such as aluminum and its alloys, with more stable metals, such as nickel, that provide enhanced solderability, bondability and corrosion resistance.
Aluminum is widely used for circuitry patterns on integrated circuit (IC) chips but is very difficult be directly solder because it is a very reactive metal that forms a thick oxide layer, which prevents direct contact between the solder and the aluminum metal. This thick oxide is recalcitrant in that it is difficult to remove and reforms almost instantaneously under ambient conditions. Consequently, electrical contact to aluminum IC pads is typically made by wire bonding, which utilizes a combination of ultrasonic vibration, pressure and thermal energy to cause a gold wire to penetrate the aluminum oxide layer and form a bond to the underlying metal. The other end of the gold wire is bonded to a pad on a substrate, or on a package (e.g., ball grid array or dual in-line package) that connects to a solderable pad or pin that is subsequently soldered to a printed wiring board (PWB) or other substrate.
Wire bonding imposes significant limitations for microelectronic applications. Since the bonds are made one at a time, the process is relatively slow and expensive compared to soldering, which can form thousands of connections almost simultaneously. The expense of the gold wire adds significantly to the costs. In addition, the fixture (head) for holding the gold wire and applying the pressure and energy needed for wire bonding has finite dimensions that limit the minimum spacing between adjacent bond sites. Another important drawback is the inductance of the gold wires themselves, which becomes appreciable at high signal frequencies and limits device switching times or clock speeds. Furthermore, because of wire length and routing issues, wire bonds are not practical for connecting to the area arrays, e.g., ball grid arrays, that are becoming commonplace as the sizes of IC chips decrease and clock speeds and the number of input/output (I/O) connections increases. Solderable IC pads are essential to the emerging flip chip technology, which provides the ultimate in performance and cost reduction. In this case, pads on the IC chip are soldered directly to pads on the substrate PWB, which eliminates the cost and limitations of a package, provides area array capability, and minimizes signal losses.
The most widely used approach for rendering aluminum IC pads solderable involves sputter cleaning/ablation to remove the aluminum surface oxide layer, and immediate sputter or vapor deposition of a layer of oxidation-resistant metal to protect the aluminum pads against re-oxidation. Such vacuum processing is inherently very expensive and also requires photoresist masking to enable lift-off of metal deposited in non-pad areas, or to confine subsequently electrodeposited metal layers to the pads. Electroplating is sometimes used to provide thicker layers of protective metal needed for reliable soldering, and/or to deposit solder that is reflowed to form solder balls for ball grid array (BGA) devices. In this case, the thin metal buss layer needed to provide electrical contact to the aluminum pads is often deposited directly on the IC surface and must be removed by etching (after photoresist removal). A significant concern for electroplating processes is non-uniformity of the plated layers, especially overplating of isolated pads and underplating of those close together.
Displacement plating from solution is attractive as a potential alternative for rendering aluminum pads solderable without the need for costly vacuum deposition, photoresist masking, and etching processes. In this case, the aluminum surface oxide would be dissolved in a solution that contains ions of a more noble metal, e.g., nickel, which would be deposited on the aluminum substrate by the electrons generated by aluminum dissolution in the solution. Since the displacement process should cease when the aluminum surface becomes completely covered with the displacement metal and is no longer exposed to the solution, the layer of deposited metal would necessarily be thin but could readily be thickened by subsequent electroless deposition. An additional thin layer of a noble metal, e.g., gold, could then be deposited by electroless or displacement plating to protect the thickened coating against oxidation and solderability loss.
Processes for direct displacement plating of copper on aluminum from aqueous alkaline or acidic fluoride baths are reported in standard handbooks (e.g., Metal Finishing Guidebook and Directory Issue, published annually by Metal Finishing magazine, Tarrytown, N.Y.). However, copper can migrate rapidly in aluminum and degrade both the mechanical properties of the aluminum and the electronic performance of the underlying silicon. Consequently, a barrier layer, e.g., titanium/tungsten or tantalum nitride, is required between copper and silicon on IC chips, which precludes the possibility of utilizing a copper displacement process. Furthermore, copper displacement coatings on aluminum tend to be porous and poorly adherent, and can produce rapid corrosion of the aluminum via galvanic action, so that they are only moderately effective for protecting aluminum against corrosion or serving as the basis for a corrosion-resistant overlayer. Note that copper is generally added to aluminum IC pads to improve electromigration resistance but the concentration is low ( less than 3%) and alloying prevents migration into the silicon.
The only displacement plating process believed to be presently available for applying a solderable metal suitable for IC chip applications to aluminum involves intermediate displacement plating of zinc from a strongly alkaline solution (zincating), and subsequent displacement of the zinc by nickel in an electroless nickel bath. In this case, the aluminum surface oxide dissolves via reaction with hydroxide ion in the alkaline zincating solution (Al2O3+2OHxe2x88x92xe2x86x922AlO2xe2x88x92+H2O) and oxidation of the underlying aluminum drives reduction of zincate ion to produce a layer of zinc metal (2Al+3ZnO22xe2x88x92+2H2Oxe2x86x923Zn+2AlO2xe2x88x924OHxe2x88x92). In a separate electroless nickel bath, the zinc layer is displaced by a nickel layer (Zn+Ni2+xe2x86x92Zn2++Ni), which is thickened by electroless nickel deposition.
This process for indirect displacement plating of nickel on aluminum via zincating has major drawbacks, especially for plating pads on IC chips. One fundamental problem is that hydroxide ion in the strongly alkaline zincating solution aggressively attacks the aluminum itself with evolution of hydrogen gas (2Al+2OHxe2x88x92+2H2Oxe2x86x922AlO2xe2x88x92+3H2). Pads that are not at least 1 xcexcm thick may exhibit bare spots or be completely consumed. In addition, the zinc deposit is porous and non-uniform since the displacement reaction must occur rapidly while dissolution and hydrogen evolution are also occurring on both the bare aluminum substrate and the zinc metal deposit. Double zincating consumes even more aluminum and only partially improves the zinc deposit quality. Furthermore, poor adhesion of the nickel deposit results if the zinc deposit is not completely removed by dissolution in the electroless nickel bath, which is difficult to ensure. Another fundamental problem with the overall process is the use of an electroless nickel bath for both the nickel/zinc displacement and electroless deposition reactions, which have dichotomous requirements. In particular, strongly complexed nickel ions should provide better quality deposits by slowing the displacement reaction to a moderate rate, but would not be reducible by the mild reducing agents needed to avoid extraneous electroless deposition. Buildup of zinc ions in the electroless nickel plating bath can also degrade the deposit quality. It is not surprising that such a dynamic process with many uncontrollable variables has not produced consistent results in production use.
A method for direct displacement plating of nickel on aluminum might be expected to yield the high quality deposits needed for a practical IC chip bumping process but has not been developed previously despite the long standing need. One reason is that the hydroxide ion employed in the prior art to remove the aluminum surface oxide in the zincating process cannot be used in a nickel displacement plating solution since it would precipitate nickel hydroxide, which is highly insoluble. Note that nickel forms amine complexes in aqueous solution that are stable in the presence of hydroxide ion but these do not allow nickel displacement plating on aluminum. Fluoride ion is known to dissolve aluminum oxides in aqueous solutions but its use as an activator for nickel displacement plating is not taught by the prior art literature. This may be because it has not been recognized that close control of fluoride activity is required in order to avoid both excessive attack of the aluminum substrate and the porous, non-uniform deposits which result from the competing gas evolution reaction (2Al+6HFxe2x86x922AlF3+3H2). The situation is further complicated because of the relatively low activity of nickel for displacement reactions. Needed means for controlling the fluoride activity are also not evident from the prior art literature.
Consequently, there is a significant need for a displacement plating process for aluminum that provides uniform, adherent deposits of a solderable metal (e.g., nickel, cobalt or palladium) that are sufficiently stable to be thickened in a separate plating bath. Such a process would provide the basis for cost-effective bumping of IC chips and could be used to provide improved corrosion protection for various highly reactive metals, including aluminum, titanium, magnesium, beryllium, and their alloys. In addition, high-quality nickel coatings on aluminum, with a thin gold overlayer to prevent oxidation of the nickel, could be used to preserve the wire bondability of aluminum pads on IC chips or to restore wire bondability to pads that had developed excessively thick oxide layers during storage or processing. Degradation in wire bondability of aluminum pads is a particular problem for low-volume IC chip users who must purchase in volume to obtain a good price and then store chips for later use. Long-term storage is also necessary to ensure availability of chips needed to support field equipment having a long service life. An effective displacement plating process might also be used to remove substrate oxidation and repair coatings that had rendered previously coated reactive metal parts unsolderable or unbondable.
The displacement plating process of this invention provides a uniform, adherent coating of a relatively stable metal (e.g., nickel) on a highly reactive metal (e.g., aluminum) that is normally covered with a recalcitrant oxide layer. This process may be used to treat a part consisting of the reactive metal or containing the reactive metal, as well as other materials that are not adversely affected by the displacement plating solution. Electroless plating or electroplating can be used to provide a thicker coating of the same or a different stable metal that is solderable, wire bondable, and/or provides enhanced corrosion protection. A thin overlayer of a noble metal, e.g., gold, may be used to suppress oxidation or corrosion of the thickened coating itself, and to provide additional protection to the underlying substrate. Coatings produced by this invention can be used to preserve or restore wire bondability for aluminum pads on IC chips and substrates, to provide or improve corrosion protection for highly reactive structural metals and alloys, and to render aluminum pads on IC chips (and substrates) solderable without the use of expensive masks and vacuum deposition processes. The solderable pads provided by this invention can be solder bumped for flip chip attachment.
The displacement plating solution of this invention comprises a solvent (e.g., dimethylsulfoxide or ethylene glycol), a fluoride activator to dissolve the reactive metal surface oxide layer, and ions of the relatively stable metal to be deposited by displacement plating on the reactive metal. Use of a nonaqueous solvent is preferred to avoid or suppress reactions between the reactive metal and aqueous species that would interfere with the displacement process. Nonetheless, appreciable amounts of water may be present without significantly reducing the benefit provided by the nonaqueous solvent. It may be beneficial to control the activity of the fluoride activator via the type and concentration of fluorine compound employed. Fluoride activity for a given concentration decreases in the order: Fxe2x88x92 greater than AlF63xe2x88x92 greater than SiF62xe2x88x92 greater than  greater than PF6xe2x88x92, BF4xe2x88x92 (determined in dimethylsulfoxide solvent). Appropriate stable metals for coating the active metal include nickel, cobalt and palladium. Stable metal ions are preferably added to the solution as compounds with halide or pseudohalide anions, which tend to inhibit the displacement process by complexing the stable metal ions and adsorbing on the substrate metal surface. Excessively fast displacement plating results in porous, poorly-adherent deposits. On the other hand, the halide/pseudohalide anions should not complex the stable metal ions so strongly that the displacement plating reaction is slowed to an impractical rate.
Uniform, adherent deposits of more stable metals on highly reactive metals are attained by displacement plating according to this invention by optimizing the rates of reactive metal oxide dissolution, substrate dissolution, and the displacement reactions via concerted control of the solution composition, temperature and agitation. Operation at an elevated temperature is typically required to provide practical rates for the oxide dissolution and displacement reactions. Dissolution of the reactive metal substrate must be minimized to avoid deposit porosity and excessive consumption of the substrate material, since the activator species needed to dissolve the oxide also attacks the active metal substrate. When the surface oxide on the reactive metal is contaminated, nonuniform or excessively thick because of prior storage or processing, more consistent results and/or more uniform displacement coatings may be obtained by a pretreatment in a cleaning/conditioning solution prior to the displacement plating process.
A key novel feature of the present invention is the utilization of an activator concentration gradient to minimize dissolution of the active metal substrate while providing practical rates for the oxide dissolution and displacement reactions. Such a concentration gradient is attained by utilizing a relatively high concentration of the displacing metal ions and a low concentration of the activator species coupled with minimized solution agitation within pores in the substrate surface oxide. Under these conditions, activator consumption in the oxide dissolution process reduces the concentration of activator species at the surface, especially within pores in the oxide layer where substrate attack by the activator occurs. Because of their relatively high concentration, the stable metal ions do not become significantly depleted so that the displacement reaction proceeds at a high rate. This also tends to minimize substrate attack by the activator species, which practically ceases once a continuous layer of the stable metal is formed. This approach involving use of a low level of solution agitation is counter to the teachings of the prior art since plating baths generally require vigorous solution agitation to avoid depletion of essential bath constituents at the part surface.
Direct displacement plating of nickel on aluminum according to this invention offers important advantages compared to the two-step process of the prior art, involving zincating from a strongly alkaline solution and subsequent displacement of the zinc layer by nickel from an electroless nickel bath. Attack of the aluminum substrate is minimized both during the displacement process and subsequently so that consumption of thin aluminum pads on IC chips is of less concern. After the uniform, adherent stable metal coating is produced, the displacement reaction practically ceases so that the process is robust, whereas the zincating reaction will continue until the pad is consumed. With the direct displacement process of this invention, attaining complete removal of the zinc displacement layer and avoiding contamination of the electroless nickel bath are not issues.